Measurement of air characteristics in the lower atmosphere

ABSTRACT

Sodar systems and methods for acoustically sounding air are disclosed in which chirps longer than 300 ms—and preferably with durations of tens of seconds—are used along with matched filter and/or Fourier processing methods to derive phase signals indicative of air characteristics in range. A listen-while-transmit strategy is preferred, the direct signal being removed by subtracting the phase signals from two or more receivers located near the transmitter so as to be in the same noise environment. The resultant differential signals can be related to cross-range wind with range distance. In one example, apparatus ( 100 ) is employed comprising a reflector dish ( 102 ) over which one central loudspeaker ( 110 ) and four microphones ( 112, 114, 130  and  132 ) are mounted, the microphones preferably being located on cardinal compass points and having their axes ( 124, 126 ) slightly angled with respect to the vertical transmission axis ( 122 ).

RELATED APPLICATION

[0001] This application is a continuation of international ApplicationSerial No. PCT/AU02/01129 filed 19 Aug. 2002, published under PCTArticle 21(2) in English; and claiming priority from Australian patentapplications PR 7203 filed 23 Aug. 2001 and PR 7832 filed 21 Sep. 2001,and applicant claims the benefit of Australian patent applications PR7203 filed 23 Aug. 2001 and PR 7832 filed 21 Sep. 2001.

TECHNICAL FIELD

[0002] This invention relates to the use of acoustic signals foratmospheric sounding and is particularly concerned with sodar techniquesfor measuring air velocity variation—such as horizontal wind speedvariation, wind-shear and/or turbulence—in the lower atmosphere. Theinvention may, however, be applied to measuring local density variationin the atmosphere, such as may be caused by temperature gradients,temperature, thermal inversions and variations in moisture content.

[0003] The apparatus and methods of the intention are also applicable towind profiling in the vicinity of airports to enhance air safety and/orpermit higher density air traffic at airports. The atmospheric soundingtechniques of the invention belong to a class of technology recentlydubbed SODAR, or Sound Direction and Ranging. Sodar is to bedistinguished from sounding techniques using electromagnetic waves, suchas RADAR (Radio Direction and Ranging), LIDAR (Light Direction andRanging), AERI (Atmospheric Emittance Radiance Interferometry) and thehybrid RASS (Radio Acousitc Sounding Systems) in the atmosphere.However, common to all these techniques in their current form is aconcern with Doppler signals and the use of Fourier transform methods inprocessing such signals. While SONAR (Sound Navigation and Ranging) hasnot been mentioned because it is employed in liquid media, some overlapbetween the exclusively acoustic techniques of sonar and sodar may beseen because sonar ranging and imaging methods have been applied inair—as in some camera ranging, non-destructive testing and medicalimaging systems.

BACKGROUND OF THE INVENTION

[0004] Though exclusively acoustic methods for wind profiling and thelike have a long history, Coulter & Kallistratova in their 1999 article“The Role Acoustic Sounding in a High-Technology Era” [Meteorol. Atmos.Phys. 71, 3-19] show that these methods have not lived up to theirpromise. This appears to have been largely due to an inability toachieve an adequate signal-to-noise ratio [s/n].

[0005] Sodars for atmospheric sounding have almost universally employshort (millisecond), single-tone high power pulses, multiple receiversand simple timing circuits to determine the sequence of echoes atdifferent receivers needed to deduce the height of variousdiscontinuities in the atmosphere. U.S. Pat. No. 2,507,121 to Sivian[1950] disclosed a method for measuring the height of atmosphericdiscontinuities that involved sending such a pulse vertically into theatmosphere and, after cessation of the transmitted pulse, detectingvertically returned echoes using two similar receivers located near thetransmitter. In the embodiment of most interest, the first receiver isshielded against receiving echoes but the second is not and the tworeceivers are connected so that their outputs are opposed and the netsignal can be displayed on an oscilloscope. In the event of a normallyreturned echo, a pip is displayed because only the second receiverdetects a signal. However, in the event of local noise such as a gunshotboth receivers detect the same signal and no pip is displayed.

[0006] U.S. Pat. No. 3,889,533 to Balser [1973] disclosed an ‘acousticwind sensor’ in which an acoustic transmitter illuminates a cylindricalcolumn of air by either CW (continuous wave) or pulsed signals andremote receivers are pointed at narrow or broad portions of two or moresides of the side column to detect acoustic energy scattered laterallytherefrom. The Doppler components of this scattered energy are thenextracted to determine wind velocity at the various heights. In order toobserve a portion of the illuminated column, which is—say—1000 m fromthe ground, the receivers need to be spaced from the transmitter by aroughly similar distance. Also, significant spacing is needed tosufficiently attenuate the direct signal [sometimes called the ‘zeroDoppler’ signal] from the transmitter. An application of this system tothe detection of persistent vortices near runways was disclosed in U.S.Pat. No. 3,671,927 to Proudian and Balser.

[0007] U.S. Pat. No. 3,675,191 to McAllister [1972] disclosed the use offour adjacent arrays of acoustic transducers capable of being used asspeakers and microphones, the arrays being aligned with the cardinalpoints of the compass and being shielded from one another, except attheir upper faces. Short acoustic pulses were transmitted verticallyupwards and the relative timing of the returned echoes at each of thefour arrays gave the height and bearing of wind layers. [It might benoted that the physics of acoustic sounding was well documented in 1969by McAllister and others in “Acoustic Sounding—A New Approach to theStudy of Atmospheric Structure” in Proc. IEEE Vol. 57, 579-587.] U.S.Pat. No. 4,558,594 to Balser disclosed the use of an acoustic phasedarray capable of directing successive pulses in different directions,the echoes from one pulse being detected by the array before the next istransmitted. U.S. Pat. No. 5,521,883 to Fage et al uses a similar phasedarray to send pulses of different frequencies in different directionsand then listen for all echoes simultaneously, thereby decreasing thecycle time. The typical angle of elevation for pulse transmission in thelatter systems was between 20 and 30 degrees. The relatively lowelevation angle is to enhance Doppler components in the returned echoesdue to horizontal (rather than vertical) wind speed in the direction ofinterrogation.

[0008] In recent years, radar DSP (digital signal processing) techniqueshave been applied to the sodar to achieve improved s/n. In particular,pulse-compression techniques have been used, in which the echoes from aphase or frequency coded pulse are processed with matched filters usingFourier transforms to give the range resolution normally associated witha shorter pulse with a much higher peak power. Such coded pulses aresaid to have ‘pulse-compression’ waveforms or to be ‘pulse coded’. Forsimplicity, pulses of this type will be called ‘chirps’. In an articleentitled: “Use of Coded Waveforms for SODAR Systems” [Meteorol. Atomos.Phys. 71, 15-23 (1999)], S G Bradley recently reviewed, withsimulations, the use of radar pulse compression techniques to improveamplitude discrimination in sodar. Examples of the use of pulsecompression techniques in radar can be found in U.S. Pat. Nos. 6,208,285to Burkhardt, 6,087, 981 to Normat et al, and 6,040,898 to Mroski et al.Despite the application of such sophisticated techniques to sodar, areview by Crescenti entitled, “The Degradation of Doppler SodarPerformance Due to Noise” [Crescenti, G. H., 1998, AtmosphericEnvironment, 32, 1499-1509], found that severe problems remain even atmodest ranges of 1500 m.

OUTLINE OF THE INVENTION

[0009] From one aspect, the invention comprises methods and apparatusfor acoustic sounding in air in which echoes from a transmitted chirp(along with extraneous acoustic inputs) are detected during transmissionof the chirp. In other words, there is ‘listening while sending’.

[0010] This avoids the need to limit pulse length to secure near-rangecapability, which is essential in known pulsed sodars that employ the‘transmit then listen’ strategy. For example if the pulse length of aconventional sodar is one second, the first 170 m of range will be lostbecause the receiver will be turned off for the first second; a 10 spulse will lose the first 1700 m of range. Typically, therefore, pulsedsodars of the art employ pulses of a few tens of milliseconds. Bycontrast, our chirps are of at least 300 ms duration and, preferablylonger than 10 s; indeed, we have used chirps of up to 50 s, theduration only being limited by our current signal processing capacity.Preferably, the duration of the chirp is at least 5% of the listeningtime; that is, there is at least 5% overlap between chirp transmissionand echo reception, but it will be appreciated that listening timedepends on the distance range covered. For ranges up to a few km, weprefer chirp lengths well over 50% of receive time. As a convenientguide, we listen for about 6 s longer than the chirp for each km ofrange. Thus, in a system with a 1 km range, the chirp/pulse durationmight be 15 s and the listening time 21 s; for a 2 km range, we mightuse a 31 s chirp and listen for 43 s. Generally, we start listening atthe commencement of the chirp transmission to obtain data from groundlevel up. For some applications however we may not want the ground leveldata and choose to start listening some time after the end of the chirptransmission.

[0011] The longer the chirp, the higher its energy for a giventransmitter power and the better the echoes can be discriminated usingappropriate matched filter and/or Fourier techniques. It is thus mucheasier to detect faint echoes behind the direct signal from thetransmitter with long, low power chirps than with conventional shorthigh-power pulses. The danger of receiver overload is also mitigated bythe use of modem microphones that have a large dynamic range. Of course,acoustic shielding of the receiver(s) from the direct transmitter signalis sensible.

[0012] Indeed, the improvement now possible with the use of long chirpsand matched filter techniques is such that, from another aspect, theinvention comprises methods and apparatus for acoustic sounding in airin which the echoes from a chirp of greater than 300 ms are detected andprocessed using matched filter and Fourier techniques. Either the ‘sendthen listen’ or the ‘listen while sending’ strategy may be used. Asalready noted, chirps with duration in the order of seconds arepreferred; with chirp durations of tens of seconds being favored in maysituations.

[0013] From another aspect, the invention comprises methods andapparatus in which multiple receivers are located near a commontransmitter so that each will receive echoes from each transmittedchirp. Preferably, the receivers are located close enough to share acommon acoustic and system noise environment and, preferably, they arearranged so as to receive the same direct signal (In both frequencyspectrum and amplitude). This allows received signal components (eg,direct signal, ground clutter and noise) that are common to more thanone receiver to be to be efficiently removed by differencing the signalsfrom two or more receiver locations. Of course, by ‘multi-receiver’ wemean to include the situation where a single receiver is moved tomultiple receiver locations and where a separate chirp is transmittedfor each receiver location.

[0014] While the removal of the common unwanted direct signal, as wellas common noise components and ground clutter, is highly desirable, itis very difficult to be done directly on the received signals for achirp that lasts tens of seconds. According to another aspect of theinvention, we employ multiple acoustic receivers with a singletransmitter and process the received acoustic signals in the (Fourier)frequency domain using matched filter techniques to generate acumulative phase output for each receiver signal and then manipulatethese outputs to achieve the appropriate measurement. Subtraction ordifferencing of the cumulative phase signals eliminates the directsignal, common noise, common ground clutter and the common signals dueto variation in vertical wind speed, the residual differentialcumulative phase then represents the variation of wind speed with rangedistance. This overcomes a major problem with conventional sodars, whichcannot discriminate between returned Doppler signals due to verticalwind speed without a direct measurement and those due to horizontal windspeed.

[0015] However, comparison or differencing of two or more cumulativephase signals requires a common starting or reference point in thesignals. This is conveniently the start of chirp transmission, which canbe determined by the start of the received direct chirp or by anelectronic signal from the transmitter. However, many other methods ofsynchronizing the receiver signals are possible. Thus, while it isdesirable that the receivers are located in a common acousticenvironment in the vicinity of the transmitter, it is not essential thatthey by equidistant from the transmitter in order to ensure that thedirect chirp arrives at each receiver at the same time.

[0016] By transmitting two differently coded chirps (at the same time,using two transmitters or one after the other using one transmitter) thecumulative phase outputs can be manipulated to remove all commonsignals, and components due to cross-range wind, to allow generation ofa further output that is indicative of variation of the speed of soundwith range and, thus, variation of temperature with range. Preferably,the two chirps are identical positive and negative linear phase chirps(eg, the positive one rising from 800 to 1600 Hz and the negative onedescending from 1600 to 800 Hz at the same phase rate.

[0017] Thus the last-mentioned aspect of the invention provides afurther large improvement in s/n, allowing much improved echodiscrimination with respect to the art, despite listening while sending.Also, simultaneous echo reception and processing by multiple receiversgreatly improves cycle time.

[0018] A convenient arrangement of receivers in a system for verticalatmospheric sounding is to locate one receiver at each cardinal compasspoint around the transmitter and to slightly incline opposed receiverstoward or away from one another. Thus, the phase components common tothe N-S receiver signals are removed by phase differencing to leave thatassociated with variation of the net N-S wind over range distance.Systems of this type are suitable for vertical sounding in noisyenvironments such as airports, power stations and urban areas.

[0019] It will be appreciated that the invention is not limited to theuse of four receivers, or to vertical sounding systems or to thesymmetrical placement of receivers around a transmitter. The receiversmay be arranged in a line, for example across a runway glide path withone or more transmitters to detect persistent vortices caused by thepassage of large aircraft. The high-speed localized winds which can makeup such vortices are difficult to quantify because the high Dopplershifts of echoes generated have considerable ambiguity. In thissituation, another aspect of the invention involves the repeatedanalysis of recorded echo signals is for each range point using amatched filter that is fed with a succession of different referencechirps so that, for each range point, a reference chirp is found thatgenerates a zero phase gradient output (in the Fourier domain). Thatreference chirp is then indicative of the wind speed at that rangepoint. It is also useful here (as well as in other applicationsenvisaged by the invention) to take ‘readings’ in the absence ofvortices to record ambient wind and noise conditions and to subtract theassociated phase signals from those generated when there is a vortexpresent

[0020] Whilst listening during sending is not essential for theimplementation of the last described aspects of the invention, it iscertainly desirable because it enables the use of long chirps, betterecho discrimination and the effective elimination of range dead-zones.

[0021] The transmitted acoustic chirp can be generated by feeding acommercially available loudspeaker (transmitting acoustic transducer)with an electrical input signal from the sound card of a computer (forexample), while the echoes can be detected using commercially availablemicrophones (receiving acoustic transducers). The loudspeaker andmicrophone(s) can be mounted with separate concentrating reflectors(plates, homs, dishes or the like) or they may be mounted with a commonreflector. For example, four microphones can be arranged in quadraturearound a single loudspeaker above a single reflector dish so that thetransmission axis is substantially axial with respect to the dish. Sincethe microphones are then offset with respect to the axis of the dish,the receiving axes of opposed pairs will be oppositely inclined towardthe transmission axis; that is, each receiver will be most sensitive toechoes coming from a direction opposite to its location on the dish withrespect to the transmission axis.

[0022] An arrangement where a loudspeaker and multiple microphones aremounted on a common structure allows the transmission axis to beconveniently aimed or set as desired by moving the structure. A systemwith a transmission axis of low elevation can be used, for example, todetect or characterize vortices caused by large aircraft landing ortaking off at an airport it is even possible to use airborne systems ofthis type to warn pilots of clear air turbulence (CAT) that is difficultto detect using radar. For example, a compact transmitter and receiversystem could be mounted in the nosecone of a large aircraft.Alternatively, only the transmitter need be mounted in the noseconesince the receivers can be mounted in a row along the leading edge ofthe wings.

[0023] As already noted, the receiving axis of a receiver may beinclined with respect to the transmission axis and that, where multiplereceivers and signal differencing are used, it is desirable that theaxes of opposed receivers are equally inclined. The optimum angle ofinclination will depend upon the aperture of the receivers, the range ofthe system and the desirability of locating the receivers in a commonacoustic environment. An angle of 20 degrees in a system with a 3 kmrange is likely to place the receivers too far from one another to havea common acoustic environment, but this may not be so in a system with arange of only 250 m. Angles of inclination of between about 2 and 10degrees have been found suitable, with angles between 4 and 7 degreespreferred. It will be seen that the point of intersection of a receiveraxis with the transmission axis is not intended to be the nominal rangeof the sodar system. Indeed, highly satisfactory results have beenobtained where medium aperture microphones are located about 1 m fromthe loudspeaker in a common dish with their receiving axes angled atabout 4 degrees to the transmission axis. In effect, the receivers of anopposed pair are looking for wind-Induced Doppler signals from largeilluminated areas on opposite sides of the transmission axis but in thesame plane as the receiving axes and the transmission axis.

[0024] As already noted, it is desirable (but not necessary) to spacemultiple receivers equidistant from and near to a common transmitter sothat each will be subject to the same ambient noise (as well as othercommon components). Generally, the louder and less uniform the noiseenvironment, the nearer the receivers need to be to one another toensure that each is subjected to the same environmental noise, as far aspracticable. We have found that, in a noisy environment, the distancebetween a receiver and the transmitter should be of the order of meters.In a quiet environment, it can be of the order of 10 m.

[0025] In general, the transmitted chirp should have a tonal range(acoustical bandwidth) suited to the object being sounded. We have foundthat wind-shear below 3000 m is best sounded at the lower end of theaudible range; for example, 500-5000 Hz, more preferably between 800 Hzand 3 kHz and most preferably between 1.0 kHz and 2.5 kHz.

[0026] While the tones in a chirp can be modulated in frequency and/orphase in many ways in conformity with pulse compression techniques, wehave found it convenient to use a linear chirp in which the frequencyincreases or decreases smoothly and linearly from start to finish of thechirp. Ideally, such a chirp has a uniform rate of phase-shift. The useof positive and negative linear chirps is of particular value in thereduction of unwanted phase components in techniques for measuring airtemperature disclosed herein. Linear chirps are also easily generatedand their echoes convenient to process using available DSP and Fouriertechniques implemented using personal computers.

[0027] Although (as already noted) long duration chirps offer thepotential of high system processing gains (lower s/n), long chirps alsoresult in significant computational demands when using the high signalsampling rates and the Fourier techniques needed to achieve such gains.We have found that current readily available FFT algorithms, DSP chipsand PCs set a practical limit on chirp duration of about 40-50 s atsampling rates of about 96 k Hz. This typically represents some 1400samples per m, given a range of 3000 m. Indeed, the computationaldemands are such that we prefer to dedicate one PC to each receiver of amulti-receiver system so that echo analysis for all receiver signals canproceed in parallel to the point where signal differencing takes place.In the future, developments in chips, FFT/matched filter techniques andPCs may allow longer chirps to be processed using a single PC—or, muchfaster updating times using the pulse lengths presently achievable.

DESCRIPTION OF EXAMPLES

[0028] Having portrayed the nature of the present invention, particularexamples will now be described with reference to the accompanyingdrawings. However, those skilled in the art will appreciate that manyvariations and modifications can be made to the chosen examples whileconforming to the scope of the invention as outlined above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In the accompanying drawings:

[0030]FIG. 1 is a series of diagrammatic plan views showing selectedarrangements of transmitters and receivers, the transmitters(loudspeakers) being shown as small shaded circles and the receivers(microphones) being shown as small unshaded circles.

[0031]FIG. 2 is a series of diagrammatic elevations showing co-locatedand separately located transmitter and receiver arrangements.

[0032]FIG. 3 is a diagrammatic sectional elevation showing thearrangement of the transmitter and receivers of the first systemexample.

[0033]FIG. 4 is a schematic plan of the system of FIG. 3 showing thegeneral manner in which signals to the transmitter are generated andsignals from the receivers are processed.

[0034]FIG. 5 is a block diagram illustrating a system for extractingeast-west phase information from the echoes received by the east andwest microphones of the chosen system example.

[0035]FIGS. 5A, 5B and 5C are block diagrams illustrating respectiveparts of a circuit and process by which wind-speed, wind-bearing andwind-shear information with respect to height can be generated with thesystem of the first example.

[0036]FIG. 6 is a histogram depicting a typical digitized acousticsignal detected by a receiver of the system of the first example.

[0037]FIGS. 7A and 7B are graphs of typical signals before and after thelow-pass filter of the matched filter of the system of the firstexample.

[0038]FIG. 8 is a series of graphs depicting the cumulative phaseinformation derived from all receivers of the system of the firstexample.

[0039]FIG. 9 includes bar charts and graphs showing the atmospheric windcharacteristics output from the system of the first example.

[0040]FIG. 10 is a block diagram illustrating portion of the process bywhich temperature with respect to height is derived

[0041]FIG. 11 is a graphical representation of how phase signals ofpositive and negative chirps are differenced to yield an indication oftemperature with respect to range distance.

[0042]FIG. 12 is a plot of temperature with respect to range distancegenerated by the use of the disclosed system.

[0043]FIG. 13 is a block diagram illustrating portion of the process bywhich vertical wind speed with respect to height is derived.

[0044]FIG. 14 is a schematic diagram illustrating the detection of wakevortices left by a large airplane near a runway.

[0045]FIG. 15 is a graph showing the notional variation of excess phasewith respect to range distance, this relationship being used to computewind velocity and height in awake vortex.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0046] As illustrated by the simple plan-view diagrams (a) to (i) ofFIG. 1, there are many possible ways in which the transmitter andreceivers of sodar systems envisaged by this invention can beconfigured, and many others are possible. Diagram (a) shows a convenientand economical configuration in which four microphones 10 are spacedwithin and around a common parabolic reflector dish 12 and a singleloudspeaker 14 is located at the central focus of dish 12. In this way,the amplitude lobe of the transmitted pulse is vertical but theamplitude lobe each received echo is angled slightly to towards the axisof dish 12 and loudspeaker 14.

[0047] Diagram (b) of FIG. 1 shows three microphones 16 evenly spaced ina common dish 18 that also has a centrally located loudspeaker 20.Diagram (c) shows a dish 22 with a central loudspeaker 24 and only oneoffset microphone 26, dish 22 being mounted so that it can be rotated tosuccessively put microphone 24 in different positions [eg, thoseillustrated in (a) or (b)]. Diagram (d) shows four microphones 28mounted in a common receiving dish 30 that is separate and spaced fromthe associated loudspeaker 32, which is mounted in its own transmissiondish 34. Optionally, receiving dish 30 may have more or less than fourmicrophones located therein. Diagram (e) illustrates a configuration inwhich each of four microphones 36N, 36S, 36E and 36W has its ownreceiving dish 38N, 38S, 38E and 38W (respectively) and a singleloudspeaker 40 has its separate dish 42. Diagram (f) is similar to (e)except only three microphones 44, 46 and 48 and their respective dishes50, 52 and 54 are deployed around a single microphone 56 and its dish58. Finally diagram (g) shows a configuration in which a singlemicrophone 60 and its dish 62 are mounted so as to be rotatable around asingle loudspeaker 64 and its dish 66, so as to be able to simulateconfigurations such as those of (e) and (f).

[0048]FIG. 1(h) is a plan view of a linear array of four pairs ofreceivers 70 a and 70 b, 71 a and 71 b, 72 a and 72 b, and, 73 a and 73b arranged in a row with one receiver of each pair positioned on eitherside of a single transmitter 74. Transmitter 74 generates a narrowspherically propagated beam orthogonal to the line of receivers so thatthe signals received by each receiver pair can be processed to removethe direct signal and common noise. In the linear array of FIG. 1(i),which is also shown in plan view, A row of transmitters 76 is arrangedparallel to a row of receivers 78, a common signal being fed to alltransmitters so as to generate a linearly propagated sound wave. Thearrangements of FIGS. 1h) and 1(i) are suited for arrangement across theglide path of an airport to detect persistent vortices. [A system ofthis nature will be described in more detail below.]

[0049] The diagrams (A), (C) and (E) of FIG. 2 are diagrammaticelevations of configurations (a), (c) and (e) respectively of FIG. 1.

[0050] The system 100 of the first example, illustrated in FIG. 3,approximates the arrangement of FIG. 1(a) and FIG. 2(A) and is adaptedfor vertical or inclined atmospheric sounding where both cross-range(horizontal, in this case) and along-range (vertical, in this case) windvelocities are required. For convenience, however, it will be assumedthat chirps from the transmitter are directed vertically upwards toilluminate an inverted cone of air indicated at 101 by broken lines.Also for convenience, the cardinal points of the compass will bereferred to as N, S, E & W, as well as north, south, east and west wherethought necessary.

[0051] System 100 includes a large main dish 102 and a small secondarydish 104 mounted directly above the main dish. A transmitter/receivermodule 106 is supported centrally above large dish 102 (by struts thatare not shown) and, in turn, supports small dish 104 on the top thereof.Module 106 comprises a sound-adsorbent molding 108, into the bottom ofwhich a central loudspeaker 110 and four peripheral microphones arefitted.

[0052] The microphones are arranged in quadrature and aligned with thecardinal points of the compass so that, in the sectional diagram of FIG.3, the W microphone is shown at 112 (on the east/right side ofloudspeaker 110) and the E microphone is shown at 114 (on the west/leftside of loudspeaker 110). This apparent reversal of naming the E and Wmicrophones is convenient because the microphone on the west side of theloudspeaker is positioned to be most sensitive to echoes coming from theeast, after reflection and focusing by dish 104, and vise versa. Theaxis of each microphone is angled to the vertical at between about 3 and10 degrees. The loudspeaker 110 and microphones 112 and 114 are locatednear the focus of large dish 102. A fifth directional microphone 116 islocated at the focus of small dish 104.

[0053] In the diagram of FIG. 3, a single horizontal reflectiveatmospheric discontinuity (such as the nocturnal boundary layer or otherthermal inversion layer—TIL), 120 is shown. Since loudspeaker 110 ispointed vertically downward, it will generate a downwardly directedvertical beam that will be reflected vertically upward by large dish 102along a central system axis 122. Some echoes will be returned down axis122 to microphone 116 on small dish 104. However, the beam ofinterrogating pulses will be conical and will illuminate a significantarea of the TIL 120 around axis 122 and echoes will be returned from anarea to the west of axis 122 along the axis 124 of west microphone 112and be most strongly detected by that microphone (in comparison with thesignals detected by the other microphones). Similarly, echoes from TIL120 to the east of axis 122 will travel along the axis 126 and be moststrongly received by east microphone 114. While echoes returned from TIL120 and detected by W microphone 112 may be centered about path 124,microphone 112 will pickup echoes from a large area of TIL 120 in thevicinity of axis 122. Thus echoes from a source near axis 122 are likelyto be picked up by all microphones. Doppler (phase) components common toechoes detected by all microphones at much the same time are thereforeindicative of the vertical velocity of TIL 120 in the vicinity of axis122, and it can be expected that these common components will be mostprominent in the echoes detected by central microphone 116. If theDoppler components of echoes received by W and E microphones 112 and 114are subtracted, the common Doppler (phase) components indicative of thevertical velocity component will be removed and it can be assumed thatthe remaining Doppler (phase) components are due to net wind speed inthe east/west direction. Similar subtraction of the Doppler componentsof echoes received by the N and S microphones will yield the net windspeed in the north/south direction.

[0054] In practice, of course, there will be many atmosphericdiscontinuities at many altitudes within range that generate echoes andthat the time of return of such echoes will be indicative of range orattitude and the amplitude of the echoes will be indicative of themagnitude of the respective discontinuities.

[0055] The generation of chirps for transmission and the processing ofreceived echoes may be implemented in many ways, whether in software orhardware. The mode of implementation will be influenced by the desiredchirp length, listening time/intended range, sampling rates, and up-datefrequency, since these factors largely determine computation demand. Toprovide desired high processing gain, chirp durations greater than 0.3 sare considered essential, with durations of tens of seconds desirable.Prior art pulsed systems using the transmit-then-listen strategytypically have transmit times in the order of tens of milliseconds and anet listening time of about 6 s for a range of 1000 m. By contrast, inthe present example, for a range of 1000 m and net listening time of 6s, the selected chirp duration is 37 s and the total listening time is43 s. For the same total transmitted energy the chirp of the presentexample can have a thousand-fold lower peak power than a 37 m chirptypical of the prior art The total listening time of the present exampleis more than seven times that typical of the art for a 1000 m range,providing much greater opportunity for processing gain. By using chirps(pulse-compression waveforms) and matched filter processing, processinggains are further multiplied many-fold.

[0056] The computation demands of the system of the example are,however, substantial and, in this case, were thought to justifydedicating a PC to process the signals from each receiver and usinganother PC as a controller. FIG. 4 shows this arrangement in which the Nand S receiver microphones are shown at 130 and 132 respectively, thededicated N, E, W & S and vertical (V) calculating PCs are shown at 134,136, 138, 140, 142 and 143 respectively and the controller PC is shownat 144. Controller PC 144 generates the chirp for transmission bytransmitter/loudspeaker 110 and the reference chirp for use in matchedfiltering by the calculating PCs. It also collects the results of thecomputations of PCs 134-142 for integration, display and reporting.

[0057] In this example, the chirp has a phase/frequency that increaseslinearly over the 37 s from 800 Hz to 1600 Hz (the chirp could just aseasily decrease linearly from 1600 Hz to 800 Hz) and is emitted at anacoustic power of a few hundred milli-Watts that remains constant forthe duration of the chirp. This type of pulse compression waveform has asmall bandwidth (about 800 Hz) and is simple to generate accuratelyusing a PC sound card and conventional loudspeaker driver circuits forpowers up to many Watts. It is also one for which a ‘matched filter’ orcorrelator can be readily designed and used to extract echoes of thechirp from received signals having high noise levels and to effectivelycompress the energy of each echo into a period of time that is muchshorter than that of the transmitted chirp. The use of pulse-compressionwaveforms and matched filters thus yield very high processing gains.

Measurement of Cross-range Wind Velocity

[0058] System 100 of the chosen example is well suited for measuringvarious characteristics of horizontal (cross-range) wind. The manner inwhich the net phase difference between the east and west echoes iseffected to derive the E-W wind speed with range distance will now bedescribed with reference to FIGS. 5A, 5B and 5C. It should beappreciated that the system of FIG. 6A is duplicated for the extractionof the net phase difference between the north and south echoes inexactly the same manner. This is indicated by “W/N” and “E/S” in FIG.5A. However, the following description will refer principally to theprocessing of the E & W echoes.

[0059] The processes of FIG. 5A are performed in dedicated E & W PCs 138and 140 and control PC 144 to (i) extract the cumulative phase andamplitude information from the noisy received signal using matchedfiltering and (ii) difference the E-W cumulative phase information toeliminate the zero Doppler signal (and other common noise components) toyield a Doppler phase signals indicative of net east-west wind speed.The cumulative phase information and the differenced E-W and N-S phaseinformation are illustrated graphically in FIG. 8. The E-W and N-Ssignals are then used as inputs for the circuit and process of FIG. 5B,from which overall wind velocity and wind shear information is derived.PCs 138 and 140 again do the processing, the results being collated anddisplayed on control PC 144 and illustrated graphical in FIG. 9. FIG. 5Cillustrates in more detail the phase and amplitude extraction circuit,which includes a matched filter or correlator, shown in FIG. 5A. Thephase extractor is substantially identical for each receiver and isimplemented in the respective receiver PC. The circuit and process ofFIG. 5 are illustrated and described generally because the techniquesemployed are analogous to known signal processing techniques used inradar.

[0060] In FIG. 5A, loudspeaker 110 is shown pointing upwards, thetransmitted chirp is indicated by shaded graph 150, east microphone 112is shown pointing up on the right side of the diagram to receive inputsignals indicated by arrow 152 and west microphone 114 is shown pointingup on the left side of the diagram to receive input signals indicated byarrow 154. Chirp 150 is generated using a control PC output 156(indicated by associated graph 169) to drive a transmitter circuit 158,which in turn, powers matched loudspeaker 110 to ensure that theacoustic power of transmitted chirp 150 is uniform over its wholebandwidth (about 800 Hz). In this example, the power, is about 0.5 Watt;not enough to cause annoyance even in urban areas.

[0061] Received east and west signals 152 and 154 are respectivelyprocessed (essentially amplified and band-pass filtered) in receivers159 and 160, the outputs of which are sampled at 96 k/s and digitized byanalog-digital (A-D) circuits 161 and 162 to generate digital receiversignals 163 and 165. A representation of signal 163 or 165 is shown inFIG. 6.

[0062] Outputs 163 and 165 are fed to phase and amplitude extractioncircuits 166 and 167 that employ matched filters to correlate signals163 and 165 in the Fourier domain with a version of the transmittedchirp, extract or derive echo phase and amplitude information therefrom.Said version of the transmitted chirp is provided via line 168 bycontrol PC 144 to each extractor 166/167 and comprises the transmittedchirp shifted down by 800 Hz; ie, converted to a 0-800 Hz chirp asindicated in nearby graph 169. It may also be desirable to delay signalon line 168 with respect to the actual transmitted signal so as toselect a desired range band of the system.

[0063] Extractor circuits 166 and 167 each have two outputs. Outputs 170and 171 of extractor 166 are respectively indicative of the cumulativephase and amplitude of the input 163 after matched filtering (Fourierprocessing), the east cumulative phase being shown in FIG. 8(i), thesouth cumulative phase being shown in FIG. 8(v) and the east and southmagnitudes being shown in the barchart of FIG. 9(i). Outputs 172 and 173of extractor 167 are respectively indicative of the cumulative phase andamplitude of the input 165 after matched filtering, the west cumulativephase being shown in FIG. 8(ii), the north cumulative phase being shownin FIG. 8(iv) and the west and north amplitudes being shown in thebarchart of FIG. 9(i). Cumulative phase outputs 170 and 172 contain theinformation from which wind velocity, direction and wind shear can bederived, as well as common phase noise and zero-Doppler components dueto the direct signal and ground clutter. Cumulative phase outputs 170and 172 are differenced in circuit 174 to remove the common componentsand output on line 176 as the net E-W phase or cumulative phase of graph(iii) of FIG. 8. These outputs contain the information from which theE-W velocity and wind shear are derived.

[0064] As already noted, the circuit and process of FIG. 5 is applied inan identical manner to the north and south signals to derive thecumulative north phase illustrated graphically by FIG. 8(iv), thecumulative south phase illustrated by FIG. 8(v) and the net north-southphase variation with height illustrated by FIG. 8(vi). For convenience,east-west phase difference output from circuit will be identified as 176E-W of and the north-south phase difference will be identified as 176N-S.

[0065]FIG. 5B illustrates how outputs 176 E-W and 176 N-S are processedin the chosen example to generate wind speed and bearing (ie, windvelocity) and wind shear magnitude with range distance (altitude, inthis case). Outputs 176 E-W and 176 N-S are fed to a combiner circuit180 from which wind bearing or direction is derived, as shown in FIG.9(ii). This is done by determining whether the E-W signal is positive ornegative and whether the N-S signal is positive or negative in order toplace the wind direction in the correct quadrant of the compass. Thus apositive E-W phase and a positive N-S phase will indicate that the winddirection is in the first quadrant, and so on. The angle in the firstquadrant can be determined by computing the relevant vector to give amore precise indication of bearing.

[0066] By employing the well known square-root of the sum of the squaresalgorithm, the magnitude of the phase signal can be determined to outputthe wind speed and wind shear magnitudes. This is done by feedingoutputs 176 E-W and 176 N-S to respective squaring circuits 182 and 184,summing the outputs of these circuits in adder 186 and deriving thesquare root in circuit 188.The resultant output on line 190 isindicative of the variation of wind speed with respect to altitude. Thisis illustrated by FIG. 9(iii).

[0067] By taking the derivative of the signal on line 190 usingdifferentiator circuit 192 the magnitude of wind shear with altitude isoutput on line 194. FIG. 9(iv) illustrates this. Signal on line 194 canbe further processed to apportion the wind shear into various magnitudebins using a differencing circuit 196 to generate a series of outputs198-202 etc, which are illustrated by the graphs of FIG. 9(v).

[0068] Referring now to FIG. 5C, the basic operation of each extractorwill now be described. Since the extractors are substantially identical,only extractor 166 for the east receiver will be described. Again itwill be appreciated that the matched filter and the phase and amplitudeextraction techniques adopted here are known to those skilled in theradar art Preferably, as also mentioned before, each extractor isimplemented in a separate PC that is dedicated to one receiver.

[0069] Extractor 166 essentially comprises a matched filter 210 that—inthe Fourier or frequency domain—matches a reference chirp (usually aversion of the transmitted chirp) to the confused time-domain echoesthat are included in input signal 163. Each digitized sample of E input163 is converted to complex form. The imaginary part (I) is generated bymixing the input with a digitized 2000 Hz sine signal 300 supplied bycontrol PC 144 using multiplier 302, and the real part (Q) is generatedby mixing the input with a digitized 2000 cosine signal 304 (alsosupplied by PC 144) using multiplier 306. The resultant I and Q signalsfor every sample taken during the listening period are fed to a complexFFT [fast Fourier transform] process 308, where all samples arepresented and processed as an array to generate the frequency domainoutputs I′ and Q′. These outputs are low-pass filtered at 310 to removethe upper side band. FIGS. 7A and 7B show the unfiltered and filteredsignal components Q′ and Q″, respectively. The filtered signals I″ andQ″ are then passed to complex multiplier 312 in which they aremultiplied with the down-converted output signal 168 from the control PC144. The result of this multiplication is then subjected to complexinverse FFT at 314 to generate real and imaginary sample-like outputsI′″ and Q′″ in the time domain.

[0070] Each ‘sample’ output I″ and Q″ from IFFT 314 is then processed toprovide corresponding phase and amplitude outputs 170 and 171. Phaseoutput 170 is generated by implementing the function Atan2(I″/Q″) inprocess 316 to yield a succession of phase values between −π and +π online 318, which is input into an unwrap process 320. The unwrap processis known and implementations are available in programs such as MatLab.Essentially, process 316 counts the number of 2π phase shifts togenerate an accumulated phase. For every transition from +π to −π (anincreasing phase) the phase accumulator is increased by 1, and viceversa. This output can then be displayed as a radian count with respectto sample number (proxy for time and distance) and displayed graphicallyas in FIG. 8. The amplitude output 171 is generated by implementing thefunction [I²+Q²]^(1/2) in process 322. The barchart (i) of FIG. 9 showsthe variation of amplitude with height, each amplitude reading beingcolor-coded to indicate magnitude. It will be appreciated that a sharpvariation in processed signal amplitude at a given height is indicativeof moisture or temperature change and not of wind shear. However,significant temperature differentials in the atmosphere will beaccompanied by wind change.

Measurement of Air Temperature

[0071] Using the system of the above example, the variation of airtemperature over range distance can be estimated by the use of positiveand negative chirps and manipulating the cumulative phase outputsgenerated. This comprises the second detailed example of the applicationof this invention.

[0072] Though not essential, the use of substantially identical positiveand negative linear chirps is highly desirable in the means for themeasurement of temperature. Consistent with the above example, apositive chirp that rises in frequency from 800 to 1600 Hz and anegative chirp that falls in frequency from 1600 to 800 Hz over a periodof 37 s will be assumed. It will also be assumed that the positive chirpis transmitted first and that the negative chirp is transmittedimmediately after the listening time of 43 s has elapsed, there alsobeing a listening time of 43 s after the negative chirp has beentransmitted. In this case, it will be convenient to separately digitizethe acoustic signals from each of the four receivers (N, S, E & W) forthe positive chirp and for the negative chirp and to then process eachin the manner describe above to extract the respective cumulative phasesignal.

[0073] The manner in which the cumulative phase signals are manipulatedwill now be described with reference to FIG. 10, in which the N, S, E &W cumulative phase signals for the positive chirp are shown as inputs300 and the N, S, E & W cumulative phase signals for the negative chirpare shown as inputs 302. Inputs 300 are added together and divided byfour in adder 304 to remove the horizontal (cross-range) wind componentsfor the positive chirp, and inputs 302 are added together and divided byfour in adder 306 to remove the horizontal (cross-range) wind componentsfor the negative chirp. The use of four receivers for temperaturemeasurement is convenient as the same system can be used for horizontalwind measurements as well, The temperature measurement can also be madeby using a single receiver pointed vertically but such a system couldnot be used for horizontal wind measurements. The outputs of adders 304and 306 are then differenced in process 308 to remove common componentsdue to direct signal, vertical wind, ground clutter and noise, leavingtemperature related cumulative phase difference. The gradient of thisdifference signal is then derived in process 310 and normalized inprocess 312, the output of this process being a measure of the relativechange of temperature with altitude (range). To calibrate this, theactual temperature near ground level is input at 314 and achart—indicated at 316—of temperature variation with altitude can begenerated. An actual chart generated by the means of the second exampleis appended as Figure.

[0074] The physical basis of temperature measurement in the system ofthis example is that the total rate of phase advance with respect todistance of a positive chirp passing through a layer of cold air will beslightly less than that for a negative chirp, the difference beingdependent upon temperature, The total rate of phase advance is made upof the sum of a dominant component due to the propagation of sound inair (nominally 14,500π radians for a distance of 1 km) and a minorcomponent due to the internal rate of phase advance within the chirp(eg, +800×2π/43 or 18 radians/s for a positive chirp and −800×2π/43 or18 radians/s for a negative chirp). In the case of a positive chirp, theinternal rate is positive and is added to the propagation rate; with anegative chip the internal rate is negative and is subtracted from thepropagation rate. Thus, the rate of increase of cumulative phase withrespect to distance is slightly less for the negative chirp than for thepositive chirp. However, when a cold layer of a fixed distance isencountered, the propagation rate of the positive chirp is slowedslightly so that the chirp takes longer to travel the distance theinternal phase advance is proportionately greater than it was whentraveling the same distance in warmer air. And, since the increasedinternal phase advance is positive, it will add slightly to the (nowslower) propagational phase advance. While the cold layer also slows thepropagation (phase rate) of the negative chirp and the internal(negative) phase advance is also increased as a result, the marginalincrease is not as great due to the slower phase advance of the negativechirp and therefore results in a smaller total cumulative rate of phasechange, resulting in a divergence of the rate of phase change withrespect to time/distance. This is illustrated in the diagram of FIG. 11.

Measurement of Down-range Wind Velocity

[0075] There are two ways of estimating down-range (in this casevertical) wind in accordance with the principles disclosed herein. Thefirst (comprising the third example) is to use the central receiver 116in a send-then-listen mode; the second (comprising the fourth example)is to use the four N, S, E & W receivers in a listen-while-sending mode.In both cases, long chirps (greater than 300 ms) are employed as taughtherein but, in the first, there is some sacrifice of low-altitude rangeand, in the second, there is some sacrifice of accuracy.

[0076] In the third example, a short chirp of about 0.5 s is sent byloudspeaker 110 and, immediately after, the incoming signals tomicrophone 116 are processed by the phase and amplitude extractionprocess described above to generate an corresponding cumulative phaseand amplitude output, from which vertical wind speed variation can beread or deduced. This method will lose the first 85 m of range and willbe subject to errors due to noise and cross-range wind speed, but thegreater resolution offered by the long chirp will be gained.

[0077] In the fourth example, a chirp of about 5 s can be employed whileall receivers are listening, the acoustic outputs of each receiver thenbeing processed and the respective phases and amplitudes extracted asdescribed above. Referring to FIG. 13, the cumulative phases of the N,S, E & W receivers, indicated at 320, are fed to adder 322 where theyare added together and divided by four, as in the temperaturemeasurement case to remove common elements due to cross-range wind.System dependent phase shift 323 is then removed in process 324, itbeing easily removed because it has a constant gradient The output ofprocess 324 is indicative of the variation of down-range wind speed, butis degraded by the (relatively short) direct signal and by groundclutter. These undesired signal components are manifest in a relativelylarge phase signal at the origin (zero distance and time) Since it isreasonable to assume that the wind speed at ground level is zero theinitial or residual phase can be subtracted from the cumulative phase inprocess 326 and normalized in process 328, yielding the displayindicated at 330. Alternatively, the resultant phase can be scaled andcalibrated according to a known wind speed at a given altitude obtainedby other means, such as radio sonde.

Detection of Vortices Near Runways

[0078] In this fourth and final example, an array 400 of receivers andtransmitters—such as indicated in FIG. 1(h) or (i)—is arranged acrossthe glide path at the end of an airport runway. FIG. 14 shows the use ofan array of the type shown in FIG. 1(h) having a single centraltransmitter 402 and multiple receivers 404 extending in a row on eachside. The entire array may span 150-200 m. In FIG. 14 transmitter 402 isshown generating a spherically propagated acoustic chirp 406 and thewake vortices are shown at 408, arrows 410 indicating the direction ofrotation of each vortex. The interaction of chirp 406 with the vorticesresults in the backscatter of echoes 412, which are picked up byreceivers 404 and fed to respective matched filters in a manner similarto that described in the first example. Multiple receivers are used hereto provide better horizontal resolution and increase overall receivergain.

[0079] The difficulty in this case is, however, that the high Dopplerechoes returned from the vortices creates considerable ambiguity in theresults so that the location, size and speed of the vortices cannot bemeasured with sufficient accuracy by using the system of the firstexample alone. It is necessary to obtain measurements of the ambientconditions prior to the arrival of a large plane and use thosemeasurements to adjust and sharpen those taken with a vortex present,after the plane has passed. The ambient measurement provides a referencefor the amplitude and phase of the system as well as for the ambientamplitude and phase. These results are stored.

[0080] When the echoes from a vortex are being processed, the signalprocessing proceeds for each receiver as described in the first example,except that the return signal is correlated against multiple differentDoppler shifts (positive, zero and negative) for the internal multiplyof the matched filter using a corresponding series of reference chirpsgenerated by the control PC. Each phase calculated in respect of thevortex condition for each receiver is then differenced with thecorresponding calculated phase for the ambient condition for eachrespective receiver, removing system and noise phase shifts. This leavesa residual or ‘excess’ phase shift that can be plotted with respect todistance (time) along with the corresponding Doppler shifts, as shown inFIG. 15.

[0081] To then estimate Doppler vs. distance (height), it is necessaryto search through each excess phase record for each Doppler shiftedresult to find the distance ranges for which the excess phase is zero.By combining the ranges for which a zero excess phase is found a graphof Doppler shift vs. distance can be made. From this it is an easymatter to estimate wind speed vs. distance. It is to be noted that theexcess phase alone cannot be used to without the additional Dopplerprocessing because the range ambiguity effect spreads the excess phaseresult out to the extent that the location and size of the vortex cannotbe sufficiently located.

[0082] Bradley (cited above) discusses a similar estimation process forthe optimization of amplitude and reference should be made to thatpaper. However, using phase is more accurate and stable than amplitude,allowing a better estimation of Doppler to be obtained.

[0083] While some examples of the application of the invention have beendescribed, it will be appreciated that the methods of the presentinvention can be applied widely to acoustic sounding and that manyalterations and additions can be made without departing from the scopeof the invention as defined by the following claims.

1. A method for acoustically sounding air over a range that extends awayfrom an acoustic transmitter and receiver, the method comprising thesteps of: transmitting an acoustic chirp comprising coded pulses havingpulse compression waveforms and having a duration of at least 300 msdown-range, using the receiver to detect acoustic inputs and to generatea receiver output that is representative of said inputs, and processingsaid receiver output to generate signal phase data indicative of aircharacteristics in the range.
 2. A method according to claim 1, whereinsaid step of using the receiver to detect acoustic inputs includesdetecting echoes returned by the chirp while the chirp is still beingtransmitted.
 3. A method according to claim 1 including the steps of:using the receiver means to detect first acoustic inputs, includingechoes returned in a first direction from the chirp, to generate a firstreceiver output related to said first inputs, using the receiver meansto detect second acoustic inputs, including echoes returned in a seconddirection from the chirp, to generate a second receiver output relatedto said second inputs, generating, using at least one of Fourier andmatched filter techniques, a first phase signal comprising phase-relatedcomponents from said first receiver output, generating, using at leastone of Fourier and matched filter techniques, a second phase signalcomprising phase-related components from said second receiver output,manipulation of said first and second phase signals to generate datarelating air characteristics in range.
 4. A method according to claim 3wherein said manipulation includes the step of: adding said first andsecond phase signals to generate a first additive phase signal thatemphasizes common components of said first and second phase signalsindicative of down range air movement and to reduce components of saidphase signals indicative of cross-range air movement.
 5. A methodaccording to claim 4 wherein said manipulation includes the step of:subtracting from said first additive phase signal a reference phasesignal indicative of system phase noise.
 6. A method according to claim3 wherein: said first acoustic inputs include a first direct-chirpsignal received direct from the transmitter having substantially no echocomponent, said second acoustic inputs include a second direct-chirpsignal received direct from the transmitter having substantially no echocomponent, and said manipulation includes subtracting said first andsecond phase signals to generate a phase output signal that issubstantially free of said first and second direct-chirp signals.
 7. Amethod according to claim 3 wherein said manipulation includes: removingphase signal components that are common to said first and second phasesignals and that are at least in part due to system noise and toacoustic noise that is common to said first and second acoustic signals.8. A method according to claim 3 wherein: said first and seconddirections are inclined substantially equally and oppositely to oneanother and fall substantially in a first plane that extendscross-range, and said manipulation of the first and second phase signalsgenerates data indicative of cross-range air movement within or parallelto said first plane.
 9. A method according to claim 3 including thesteps of: using the receiver to detect third acoustic inputs, includingechoes returned in a third direction from the transmitted chirp, togenerate a third receiver output related to said third inputs, using thereceiver to detect fourth acoustic inputs, including echoes returned ina fourth direction from the chirp, to generate a fourth receiver outputrelated to said fourth inputs, generating, using at least one of Fourierand matched filter techniques, a third phase signal comprisingphase-related components from said third receiver output, generating,using Fourier techniques, a fourth phase signal comprising phase-relatedcomponents from said fourth receiver output, and manipulation of saidthird and fourth phase signals to generate data relating aircharacteristics in range.
 10. A method according to claim 9 wherein:said third and fourth directions are inclined substantially equally andoppositely to one another and fall substantially in a second plane thatextends cross-range, and said manipulation of the third and fourth phasesignals generates data indicative of cross-range air movement in saidsecond plane, and said manipulation of the third and fourth phasesignals generates data indicative of cross-range air movement within orparallel to said second plane.
 11. A method according to claim 10,wherein said first plane and said second planes are substantiallyorthogonal to one another, and said manipulation including the steps of:differencing said first and second phase signals to remove phase signalscommon thereto and to generate first differential phase componentsindicative of air movement in or parallel to said first plane,differencing said third and fourth phase signals to remove phase signalscommon thereto and to generate second differential phase componentsindicative of air movement in or parallel to said second plane.
 12. Amethod according to claim 11 wherein said manipulation includes the stepof combining the first and second differential phase signals to generatephase signals indicative of at least one of the bearing of cross-rangewind relative to the down-range direction and phase signals indicativeof cross-range wind shear.
 13. A method according to claim 11 wherein:the range extends substantially vertically from the transmitter andreceiver means, which are located near at or near the base of the range,the first and second planes are substantially vertical, the first planeextends cross-range in a north-south alignment, the second plane extendscross-range in an east-west alignment, and said manipulation of saidfirst, second third and fourth phase signals generates data indicativeof the variation of the compass bearing and velocity of cross-range airmovement.
 14. A method according to claim 9 wherein said manipulationincludes the step of: adding said first, second, third and fourth phasesignals to generate a second first additive phase signal that emphasizescommon components of said first, second, third and fourth phase signalsindicative of down range air movement and to reduce components of saidphase signals indicative of cross-range air movements.
 15. A methodaccording to claim 14 wherein said manipulation includes the step of:subtracting from said second additive phase signal a reference phasesignal indicative of system phase noise.
 16. A method according to aclaim 1 wherein the chirp is a positive or negative linear acousticsignal that has an increasing or decreasing phase or frequency, orwherein both positive and negative linear chirps are employed.
 17. Amethod according to claim 16 including the steps of: transmittingpositive and negative chirps in sequence, deriving respective positiveand negative versions of said receiver outputs, processing said positiveand negative receiver outputs to generate corresponding positive andnegative signal phase data, differencing said positive and negativesignal phase data to generate third differential data indicative ofvariation of air temperature with range distance.
 18. A method accordingto claim 16 including the steps of: simultaneously transmitting positiveand negative chirps that do not employ the same acoustic tones, derivingrespective positive and negative versions of said receiver outputs,processing said positive and negative receiver outputs to generatecorresponding positive and negative signal phase data, differencing saidpositive and negative signal phase data to generate fourth differentialdata indicative of variation of air temperature with range distance. 19.A method according to claim 17 including the step of: differentiatingsaid respective third or fourth differential phase signal to derive agradient signal that is indicative of the variation of air temperaturewith range distance.
 20. A method according to claim 1 wherein saidprocessing step includes generating ambient signal phase data in theabsence of an air disturbance at a location, and including the steps of:generating disturbance signal phase data in the presence of the localair disturbance at said location, and using said ambient signal phasedata to normalize the disturbance signal phase data and to therebygenerate normalized signal phase data.
 21. A method according to claim20 including the steps of: correlating successive samples of saidnormalized phase data against multiple Doppler values to generate dataindicative of wind speed with respect to distance.
 22. A methodaccording to claim 1 wherein signal amplitude data is generated and usedtogether with said signal phase data.
 23. A method according to claim 1wherein the duration of the chirp is greater than five seconds.
 24. Asystem for acoustically sounding air over a range that extends away froman acoustic transmitter and receiver: a transmitter adapted to transmitan acoustic chirp comprising coded pulses having pulse compressionwaveforms and having a duration of at least 300 ms down a range thatextends away from the transmitter, a receiver located near saidtransmitter and adapted to detect acoustic signals including echoes ofthe transmitted chirp returned from down-range and adapted to generate areceiver output that is representative of said received acousticsignals, and a digital signal processor for processing said receiveroutput to generate signal phase data indicative of air characteristicsin the range.
 25. A system according to claim 24 wherein: said receiveris adapted to detect a direct non-echo signal from the transmitter whileit is transmitting, said direct signal contributing to said receiveroutput.
 26. A system according to claim 24 wherein: said receiver isadapted to detect first acoustic inputs, including echoes returned in afirst direction from the chirp, and to generate a first receiver outputrelated to said first inputs, said receiver is adapted to detect secondacoustic inputs, including echoes returned in a second direction fromthe chirp, and to generate a second receiver output related to saidsecond inputs, said signal processor is adapted to receive said firstand second receiver outputs, process said outputs using at least one ofa matched filter and a Fourier processor, generate respective first andsecond phase signals, and manipulate said first and second phase signalsto generate data relating air characteristics in the range.
 27. A systemaccording to claim 26 wherein the signal processor, when manipulatingsaid first and second phase signals, is adapted to: add said first andsecond phase signals to generate a first additive phase signal thatemphasizes common components of said first and second phase signalsindicative of down range air movement and to reduce components of saidphase signals indicative of cross-range air movement.
 28. A systemaccording to claim 27 wherein the signal processor, when manipulatingsaid first and second phase signals, is adapted to: subtract from saidfirst additive phase signal a reference phase signal indicative ofsystem phase noise.
 29. A system according to claim 26 wherein thesignal processor, when manipulating said first and second phase signals,is adapted to: subtract said first and second phase signals to generatea phase output signal that is substantially free of direct-chirp signalcomponents.
 30. A system according to claim 26 wherein the signalprocessor, when manipulating said first and second phase signals, isadapted to: remove phase signal components that are common to said firstand second phase signals and that are inter alia due to system noise andto acoustic noise that is common to said first and second acousticsignals.
 31. A system according to claim 26 wherein: said first andsecond directions are inclined substantially equally and oppositely toone another and fall substantially in a first plane that extendscross-range, and the signal processor, when manipulating said first andsecond phase signals, is adapted to generate data indicative ofcross-range air movement within or parallel to said first plane.
 32. Asystem according to claim 26 wherein: said receiver is adapted to detectthird acoustic inputs, including echoes returned in a third directionfrom the transmitted chirp, to generate a third receiver output relatedto said third inputs, said receiver is adapted to detect fourth acousticinputs, including echoes returned in a fourth direction from the chirp,to generate a fourth receiver output related to said fourth inputs, saidsignal processor is adapted to use at least one of Fourier and matchedfilter techniques to generate a third phase signal comprisingphase-related components from said third receiver output, said signalprocessor is adapted to use at least one of Fourier and matched filtertechniques to generate a fourth phase signal comprising phase-relatedcomponents from said fourth receiver output, and said signal processoris adapted to manipulate said third and fourth phase signals to generatedata relating air characteristics in the range.
 33. A system accordingto claim 32 wherein: said third and fourth directions are inclinedsubstantially equally and oppositely to one another and fallsubstantially in a second plane that extends cross-range, and saidmanipulation of the third and fourth phase signals is adapted togenerate data indicative of cross-range air movement in said secondplane, and said signal processor is adapted to manipulate the third andfourth phase signals to generate data indicative of cross-range airmovement within or parallel to said second plane.
 34. A system accordingto claim 33, wherein said first plane and said second planes aresubstantially orthogonal to one another, and wherein: said signalprocessor is adapted to: difference said first and second phase signalsto remove phase signals common thereto and to generate firstdifferential phase components indicative of air movement in or parallelto said first plane, and difference said third and fourth phase signalsto remove phase signals common thereto and to generate seconddifferential phase components indicative of air movement in or parallelto said second plane.
 35. A system according to claim 34 wherein saidsignal processor is adapted to combine the first and second differentialphase signals to generate phase signals indicative of the bearing ofcross-range wind relative to at least one of the downrange direction andphase signals indicative of cross-range wind shear.
 36. A systemaccording to claim 34 wherein: the range extends substantiallyvertically from the transmitter and receiver, which are adapted to belocated at or near the base of the range, the first and second planesare substantially vertical, the first plane extends cross-range in anorth-south alignment, the second plane extends cross-range in aneast-west alignment, and said signal processor is adapted to manipulatesaid first, second third and fourth phase signals to generate dataindicative of the variation of the compass bearing and velocity ofcross-range air movement.
 37. A system according to claim 33 whereinsaid signal processor is adapted to add said first, second, third andfourth phase signals to generate a second first additive phase signalthat emphasizes common components of said first, second, third andfourth phase signals indicative of down range air movement and to reducecomponents of said phase signals indicative of cross-range airmovements.
 38. A system according to claim 36 wherein said signalprocessor is adapted to subtract from said second additive phase signala reference phase signal indicative of system phase noise.
 39. A systemaccording to claim 24 wherein the transmitter is adapted to transmit achirp comprising a positive or negative linear acoustic signal that hasan increasing or decreasing phase or frequency, or wherein both positiveand negative linear chirp's are employed.
 40. A system according toclaim 39 wherein: the transmitter is adapted to transmit positive andnegative chirps in sequence, the receiver is adapted to generaterespective positive and negative versions of said receiver outputs fromacoustic input signals including said positive and negative chirps andechoes thereof, said signal processor is adapted to: process saidpositive and negative receiver outputs to generate correspondingpositive and negative signal phase data, and difference said positiveand negative signal phase data to generate third differential dataindicative of variation of air temperature with range distance.
 41. Asystem according to claim 39 wherein: said transmitter is adapted tosimultaneously transmit positive and negative chirps that do not employthe same acoustic tones, said receiver is adapted to generate respectivepositive and negative receiver outputs, said signal processor is adaptedto: process said positive and negative receiver outputs to generatecorresponding positive and negative signal phase data, and differencesaid positive and negative signal phase data to generate fourthdifferential data indicative of variation of air temperature with rangedistance.
 42. A system according to claim 40 wherein: said signalprocessor is adapted to differentiate said (respective) third or fourthdifferential phase signal to derive a gradient signal that is indicativeof the variation of air temperature with range distance.
 43. A systemaccording to claim 24 adapted to: generate ambient signal phase data inthe manner claimed in the absence of an air disturbance at a location,generate disturbance signal phase data in the presence of the local airdisturbance at said location, and use said ambient signal phase data tonormalize the disturbance signal phase data and to thereby generatenormalized signal phase data.
 44. A system according to claim 43 whereinsaid signal processor is adapted to correlate successive samples of saidnormalized phase data against multiple Doppler values to generate dataindicative of wind speed with respect to distance.